Glossary
TTY
The terminal device a process is attached to — or is not.
TTY (from "teletypewriter") is the kernel's name for a terminal device. Interactive programs check whether their output is a TTY to decide behavior: ls prints columns and colors to a terminal but one plain name per line into a pipe; many tools disable progress bars when not on a TTY.
Processes without a TTY — cron jobs, daemons, scripts run by CI — behave differently for exactly this reason. Type tty to see yours.
See it in practice
$ ls | catNotice ls prints one name per line here: it detected that stdout is a pipe, not a terminal, and changed format accordingly.
Where you'll meet it
The window that displays a shell session.
lsList the contents of a directory, with options for hidden files, long format, sorting and more.
jobs, & and nohupRun things in the background, juggle them, and keep them alive after logout.